1. Field of the Invention
The present invention relates to an optical transmission system and to a synchronization method using a time reference pulse. More concretely, the present invention relates to an optical transmission system with an optical terminal unit in network side and a plurality of optical terminal units in user side, in which the optical terminal unit in network side controls up-signals from the optical terminal units in user side in Time Division Multiplexing Access (TDMA) manner, and to the synchronization method using a time reference pulse.
2. Description of the Related Art
As for an optical transmission system in which an optical terminal unit in network side controls up-signals from optical terminal units in user side in TDMA manner, a Passive Optical Network (PON) system is known.
The PON system includes an Optical Line Terminal (OLT) arranged at a center station, a plurality of Optical Network Units (ONUs) arranged in user sides, and a star coupler optical fiber transmission line for connecting the OLT with the plurality of ONUs. This PON system locates one or more optical couplers on an optical fiber transmission line to use a part of the optical fiber transmission line in common by all ONUs. In such PON system, it is necessary to synchronize the time in OLT with that in each ONU for uplink signal transmission.
US patent publication No. 2007/0280108 A1 discloses, although it is not a PON system, time-synchronization between subscriber terminals and a subscriber accommodation router using Network Time Protocol (NTP) that is a time-synchronization protocol of network base.
Japanese patent publication No. 2007-166278 discloses, although it is not a PON system, frequency-synchronization and time-synchronization between a master station apparatus and a slave station apparatus using NTP that is a time-synchronization protocol of network base or Precision Time Protocol (PTP).
Japanese patent publication No. 2007-235371 discloses a time-synchronization method under the standard of IEEE802.3ah™ concerning a PON system. Namely, this publication discloses that a PON counter of terminal devices is subjected to drift adjustment so as to lessen a time lag between a timestamp and the PON counter, at a point of time when the timestamp contained in the control frame of down-signals is sent, and that the transmission timing of an up-signal is determined on the basis of the adjusted PON counter.
Japanese patent publication Nos. 2008-042699 and 2006-005673 disclose time-synchronization techniques compliant with multi-rates in the PON system. Namely, in Japanese patent publication No. 2008-042699, it is described that when a time synchronization frame is time-division-multiplexed over a data frame received from a non-PON side and it is transmitted to a PON side by setting a time information to it, influence given to the time information to be set is avoided by setting the time information after the frame space is adjusted. In Japanese patent publication No. 2006-005673, it is described that, in the ONU, a timer is started to count when extracting a timestamp from a control packet and the timestamp value extracted at this time is referred as a standard of output timing of up-data if the difference between the counted value of the timer and that of the timer started to count in response to the last extracting of a timestamp is smaller than a threshold.
In the PON system, since the TDMA scheme is used, there occurs no problem when down-signals are transmitted from OLT. However, when transmitting up-signals from ONUs, it may be necessary for each ONU to wait until a time slot assigned for the ONU itself. This causes that the delayed amount cannot be fixed and therefore it is difficult to rapidly obtain time-synchronization.